Molecular hybridizations of polysomal poly A-containing messenger RNAs of undifferentiated (S) and differentiated (P) mouse neuroblastoma cells with their complementary DNAs (cDNAs) had shown that there were many messenger sequences which were unique to each differentiation step. This year has been devoted to preparations for molecular cloning of cDNAs made from messenger RNAs of S and P cells, using recombinant DNA techniques. Differentiation-step-specific clones will be identified by the use of 32p-labeled unique cDNA sequences purified by recycling hybridizations.